Dead Battery Blues

Starting a car that has a dead battery

When you’ve allowed your car’s battery to die, the time-honoured way to start it again is to either push-start the car, or connect two jumper leads from a good battery to your car’s battery. Both methods carry a certain amount of risk.


Push-starting is not recommended on cars fitted with a catalytic converter. If the engine is reluctant to start it will cause unburnt fuel to reach the hot catalytic converter, and destroy the insides. If you put the gearbox into first or reverse gear, instead of second gear, just before you lift your foot off the clutch, there’s always the risk that you may damage the gearbox or final drive gear set. (I’ve done that!)


Using jumper leads is also risky. A modern car carries currents as small as 15 milli-amps in some of the computer circuits, making it easy for a stray current to damage some component.


If you’re going to use the jumper leads, make sure they’re connected correctly. The cars shouldn’t touch, and the positive (red) terminal of the donor battery should be connected to the positive terminal of the dead battery. When this is done, the negative terminal of the donor battery should be connected to some metal part on the engine of the car with the dead battery and NOT onto the negative terminal of the dead battery.


This is done because an overcharged battery will produce oxygen and hydrogen, and the addition of a small spark – may occur if the earth cable was hooked onto the negative terminal – is certain to cause the battery to explode.


When this is done the donor engine should be started and a few minutes later an attempt can be made to start the other engine. Remove the cables in reverse order.


Reviving a dead battery      

A dead battery cannot be revived by driving the car around the block a few times. A modern car battery, often called a starter battery, is constructed with many thin plates that enable it to deliver the high amperage that a starter needs to get the engine going.


Unfortunately this construction also means that the car’s alternator cannot fully recharge the battery, no matter how far you drive. In the long term this means that if a battery suffers more than five or six heavy discharges its life will be drastically shortened.


The only way to charge this kind of battery is to have it professionally charged by somebody using a multi-stage charger.


Disconnecting a bettery

A battery should not be disconnected when you’re going away for some time. Modern batteries and electronic devices are likely to leave you in the lurch if you disconnect.


Most modern engine control units require a constant source of power. When you disconnect the battery, the control unit may lose its memory and may have to be reset by a dealer. On reconnection, the entertainment system may play dead unless you supply a code.


If you have to disconnect the battery for repairs to some other component, or to replace it with another, you should use a memory minder to ensure a constant source of power. This is a battery that usually plugs into the cigarette-lighter socket to keep the computer alive while you disconnect the main battery.


A fully charged battery will normally last for several weeks. If you go away for longer you should connect a good quality maintenance trickle charger that will leave the battery fully charged without causing any damage.